Mussone Paolo G, Ip Andy W F, Schroeder Sven L M, Murray Brent S, Miller Aline F
Molecular Materials Centre, School of Chemical Engineering and Analytical Science, and Molecular Materials Centre, School of Chemistry, University of Manchester, Sackville Street, Manchester M60 1QD, U.K.
Langmuir. 2007 Mar 27;23(7):3766-73. doi: 10.1021/la0627361. Epub 2007 Feb 27.
The collapse of Langmuir monolayers of poly(vinyl stearate) (PVS) at the air-water interface has been investigated by combined measurements of the surface pressure-area isotherms and Brewster angle microscopy (BAM). Atomic force microscopy (AFM) has been used to gain out-of-plane structural information on collapsed films transferred onto a solid substrate by a modified version of the inverse Langmuir-Schaefer deposition method. At high areas per monomer repeat unit, BAM imaging revealed that the films are heterogeneous, with large solidlike domains (25-200 mum in diameter) coexisting with liquidlike domains. Upon film compression, the domains coalesced to form a homogeneous monolayer before the film collapsed at constant pressure, forming irreversible three-dimensional (3D) structures. BAM images showed that two 3D structures coexisted: buckles of varying width extending across the surface and perpendicular to the direction of the compression and dotted islandlike structures. Upon expansion, the film fractured and both 3D protrusions persisted, explaining the marked hysteresis recorded in the Langmuir isotherms. Experiments with AFM confirmed the 3D nature of both protrusions and revealed that many buckles contain substructures corresponding to narrow buckles whose heights are a multiple of a single bilayer. Additionally, many multilayer islands with diameters spanning from 0.2 mum to over 3.5 mum were characterized by varying heights between 2 nm and up to over 50 nm. The key to the formation of the irreversible 3D structures is the presence of large inhomogeneities in the PVS monolayer, and a generalized phenomenological model is proposed to explain the collapse observed.
通过表面压力-面积等温线和布鲁斯特角显微镜（BAM）的联合测量，研究了聚（硬脂酸乙烯酯）（PVS）在空气-水界面的朗缪尔单分子层的崩塌情况。原子力显微镜（AFM）已被用于通过逆朗缪尔-谢弗沉积法的改进版本，获取转移到固体基质上的崩塌薄膜的面外结构信息。在每个单体重复单元的面积较大时，BAM成像显示薄膜是不均匀的，有大的类固体区域（直径25 - 200μm）与类液体区域共存。在薄膜压缩过程中，这些区域合并形成均匀的单分子层，然后薄膜在恒定压力下崩塌，形成不可逆的三维（3D）结构。BAM图像显示两种3D结构共存：宽度不同的褶皱横跨表面并垂直于压缩方向延伸，以及点状岛状结构。在薄膜膨胀时，薄膜破裂，两种3D凸起都持续存在，这解释了朗缪尔等温线中记录的明显滞后现象。AFM实验证实了两种凸起的3D性质，并揭示许多褶皱包含与窄褶皱相对应的子结构，其高度是单个双层的倍数。此外，许多直径从0.2μm到超过3.5μm的多层岛的特征是高度在2nm到超过50nm之间变化。不可逆3D结构形成的关键是PVS单分子层中存在大的不均匀性，并提出了一个广义唯象模型来解释所观察到的崩塌现象。
